Actually.... Snopes may call "BS" on it later but I wouldn't discount the possibility. It's not like he was standing on it and suddenly fell on his ass after all. So he didn't have that hint.
Otherwise there was pain and significant trauma. The body does all sorts of things at those moments to get through. His mind is on the bike. You know he's thinking "damn, glad I didn't lay it down" when he realizes he just banged off something and was still up. And you don't really think about being awake and upright if you'd just cut off your freaking leg.
A field medic in a foreign uniform is a buddy. He's told stories of guys not having a clue what bits they were missing and they were awake and had had time to actually stop and think about it. This poor guy was still busy riding. It may not be true. But would have to give it a check mark in the "possible" column.